A typical optical disk system contains a number of adjustment points that affect operation of the system. Examples of these include adjustment for the read timing, i.e. the points in time at which the read circuitry detects or stores values of the read signal, write timing, i.e. points in time at which the write circuitry initiates a write pulse and focus offset, i.e. a signal for adjustment of the lens or other focusing system which is intended to be added to a focus servo signal such that the resulting summed signal is operative to focus a read or write beam on the storage medium. Proper adjustment of such adjustment points is required for reading and writing optical disk marks, i.e. optically detectable characteristics of small areas of the disk which can be used to store bits of data. Items which can be used as marks can include holes, pits, amorphous areas, or magnetized areas.
Typical optical disk systems contain one or more servo systems to maintain clock synchronization or focus fineness. Examples include systems described in U.S. Pat. No. 4,290,122 issued Sept. 15, 1981 to Bates, et al., U.S. Pat. No. 4,257,709 issued Mar. 24, 1981 to Mostyn, Jr., and U.S. Pat. No. 4,238,843 issued Dec. 9, 1980 to Carasso, et al. Such systems typically are not useful for providing a calibration of the respective systems because they operate only within a narrow "capture" range and thus the systems must be calibrated before such servo systems become operable. Furthermore, such systems are designed for continuous operation to maintain an already established state of synchronization or focus and thus must operate on a rapid time-basis so as not to interfere with the recording or extraction of data which is occurring simultaneously or on a time-shared basis with such servo systems. Therefore, optical disk systems require establishment of offset values or adjustment of certain adjustment points to bring the system within minimum tolerances which will enable the desired recording or extraction of data and the associated operation of servo systems.
Adjustment of such offset values or adjustment points is typically done at the manufacturer's facility before shipment to a customer or upon installation of the system at the customer's site or during service of the equipment at the customer's site. The adjustment is typically done manually by a technician or service person. Such manual adjustment requires specialized equipment and the services of a skilled technician and thus requires technician training. Because of the expense and difficulty of such manual adjustment, such adjustment is typically done only when the equipment is shipped or installed or when specifically required because of a failure of some part of the system. However, even when the initial adjustment is done correctly, changes in the components of the system such as deterioration of or changes in electronic components, or misalignment of mechanical or optical components from vibrational, thermal or other sources, will cause the original adjustments to be less than optimal after a period of use. The less-than-optimal adjustment settings can contribute to an increase in error rates or other difficulties which may be simply tolerated until such time as the error rate reaches a level where service becomes necessary and the adjustment points are readjusted by a service person.
Adjustment is typically achieved by directly connecting test equipment such as an oscilloscope to particular lines or points in the circuitry. The technician or service person then views the output of the test equipment, such as the trace of a signal on an oscilloscope, while the system is operating. The technician has been trained to recognize which aspects of the test equipment output indicate a properly adjusted parameter and which types of outputs indicate that an adjustment is needed. Based on his training and experience, the service person then makes an adjustment to the system by, for example, adjusting a delay line in the read or write circuitry or adjusting the focus offset. This process is repeated until the technician, relying on his judgment and experience, is satisfied that a proper adjustment has been accomplished.
Typically, previous calibration processes were conducted in the context of a continuous servo system, as opposed to a servo sector or servo byte system. In a continuous servo system, a timing or clock signal is pre-recorded onto the disk. A common method of providing such timing signal was to provide grooves with periodically, e.g. sinusoidally, varying depth. Typically, such systems do not have particular sectors or areas of the disk which are reserved, i.e. in which the position or absence of marks is known. Therefore, calibration was conducted using not marks, but rather, using the pre-recorded clock signal. The phase of the pre-recorded clock signal could be detected and the timing of a separately generated read clock signal could be adjusted, for example, by manually selecting a delay tap in a delay circuit to shift the generated read clock signal to a desired phase relationship with respect to the pre-recorded clock signal.
As an example, a technician typically would connect the leads of an oscilloscope to obtain values indicative of the magnitudes of points on the prerecorded clock signal. These points were selected in relationship to the read clock signal, for example, points lagging a read clock signal edge by one-half cycle and points leading the edge by one-half cycle. It was known that if the pre-recorded signal peaks corresponded in time to the read clock signal edges, such leading and lagging points on the pre-recorded signal would have substantially the same magnitude. The technician would compare the signals indicating leading points and lagging points and if these values were not substantially equal, would manually change the delay line tap until these values became substantially equal.
A number of difficulties were consequent upon this method of calibration. First, because the position of marks on the disk is unknown in a continuous servo system, it was typically necessary to install a new unwritten disk into the system before conducting the read timing calibration in order to be sure the signal indicated only the pre-recorded clock signal and not marks. Because replacement of the disk was typically required, the calibration could not be conducted automatically but required the services of a technician. Secondly, the previous calibration method required that a clock signal be pre-recorded on the disk. Furthermore, such a pre-recorded clock signal typically provides a low amplitude signal with a relatively low signal-to-noise ratio. Because of the low amplitude of the signal, in order to obtain measurable and useful values, the distance from the leading and lagging points to the pre-recorded peak was typically less than the distance between the center of the mark positions, i.e. the positions on the disk where, according to the code or scheme being used, marks can be written on the disk. For this reason, it was not feasible to use the mark reading circuitry in the calibration system. Rather, it was necessary to use special test or calibration equipment or circuitry. Because calibration required the services of a technician, there was no purpose or advantage in using other than manually-adjustable delay circuits, and in particular, there was no reason to develop or use a programmable delay.
Thus, previous read calibration systems could not be conducted automatically but required the services of a technician, were conducted in the context of a disk with a pre-recorded clock signal, required a disk in which at least an area was known to be unwritten, and typically required installation of a clean or unwritten disk, and required the use of special test or calibration equipment and could not use the already-available mark reading apparatus or circuitry.
Previous write timing calibration methods also had a number of undesirable features. In the context of a continuous servo system, write calibration was conducted by using write clock signal to write marks into a first area or sector of the disk. The marks were then read and the amplitude of the reflected signals noted. Next, the delay taps were manually adjusted to provide a different write clock phase, marks were written into another area of the disk, and the resulting amplitude was noted. The process was repeated until it was found which delay amount resulted in the greatest decrease in reflectance, i.e. the minimum reflectance. Thus, according to previous methods, sectors of the disk were written onto or sacrificed each time new delay line configurations were tested. A single calibration could require writing on and sacrifice of a plurality of disk sectors. For this reason, it was infeasible to conduct a write timing calibration on a frequent basis, for example, on each power up of the system. Furthermore, it was necessary to conduct the write timing calibration only when a disk had been installed which was known to contain unwritten sectors and usually required installation of a new or unwritten disk.
The initial set up for calibration of the focus setting was typically conducted in previous systems by monitoring the output of the optical detector, such as a quadrature-type detector, while the focus setting was adjusted. Starting from an optimally adjusted focus setting, the focus setting can be adjusted away from the optimal in either of two directions. Typically, a technician would adjust the focus setting in a first direction until a drop off in amplitude at the optical detector was noted and the focus adjustment at this drop off was noted. Next, the technician would adjust the focus in the second or opposite direction until, again, a drop off in the detected signal at the optical detector was obtained and the focus adjustment at this point was again noted. The technician would then set the focus adjustment or offset to a position about midway between the two noted positions, i.e. midway between the points at which the optical detector displayed a drop off in the two focus adjustment directions.
As with the read and write adjustments, the focus adjustment had to be conducted on a portion of the disk which was known to be unwritten, and usually required installation of a new or unwritten disk. For this reason, the focus calibration could not be conducted automatically. Furthermore, this focus calibration method did not directly measure or necessarily result in the sharpest focus, but rather, merely represented a midpoint between two unfocused positions.
Previous optical systems have provided indications of signal strength in an analog fashion, i.e. in which an electrical signal, usually continuous, is provided which is analogous to or provides an indication of the status of another system, such as reflectance of a light signal. In order to work with such systems to adjust the adjustment points as described, relatively expensive equipment and circuitry is required such as sample and hold circuits, high speed comparators and/or oscilloscopes. Sample and hold circuits commonly used in processing analog signals suffer from common deficiencies including a "droop" or drop off of the output signal with time, and "offset errors" which occur when several sample and hold devices are used which differ slightly from each other in response characteristics. Further, analog systems are most conducive to sampling a single bit position at a time and in order to produce an indication of an average value require relatively expensive analog averaging circuitry.
Thus, several advantages could be obtained from the development of a calibration apparatus and method which involves self-calibration, i.e. automatic calibration without the need for judgment or adjustment by skilled technicians. In that regard, the present invention is directed to providing method and apparatus for calibrating adjustment points in an optical disk system which can be conducted by components of the system itself at any time calibration is desired without the need for adjustment by a skilled technician.